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Yes, you can create a mksysb image on DVD (smitty mkcd), you have to use a
DVD-RAM media. I have used the ones provided by IBM that comes in a Case
and the ones provided by Maxell (DVD-RAM). I haven't try with DVD-RW media
yet.

Yes, you can use the dvd ram drive for mksysb backups. Be aware that the
backups to dvd media on the pSeries can take a few hours to complete though.
The media requirements are: Type 2 dvd-ram media 4.7Gb (single sided) or
Type 2 dvd-ram media 9.4Gb (double sided). You can use the smit mkdvd
command to create the dvd-ram based mksysb.

Be aware also that mksysb to DVD requires enough space on the rootvg disks to hold the image. The mkdvd command will create three filesystems, build the image and then transfer the image to the dvd before removing the new filesystems. If you break out of the process before it is completes you will have some manual cleanup to do.